Question

if photon don't have mass , why do photons have momentum

Answer 1

and why do photon can be attracted by sun's gravity?

Answer 2

Due to einsteins mass energy equivalence principle:

E=mc^2

energy is equivalent to mass and vice-versa.

Answer 3

thanks

Answer 4

Here's what I think -
Photons do have mass. After all, they are used to describe the particle-nature of light. I recall reading somewhere, that as your velocity gets closer to that of light, your mass becomes negligible. I comprehend this as, the mass doesn't really decrease or 'disappear'. It can be more like, mass is not being 'felt'. What I mean is, what gives us the sensation of mass? weight, ie., gravity, right? Because we have mass, gravitational force is being exerted on us. We feel that pull, hence being aware of our mass. Now, it can be that, since the photons have extremely high kinetic energy, they are able to escape the gravitational phenomenon. Its not that they don't have mass; more like, because of their high velocity, they are not 'aware' of their mass ( 'cause of no gravitational pull).
This is a possibility, is it not?

Answer 5

And as for the fact of 'bending of light', I think Einstein's theory of relativity takes care of that. Light's path in space is always linear. But because of the deformation of space (caused by massive bodies), that path also bends. That explains why light bends around sun.

Answer 6

photons don't have 'rest' mass...so their mass is associated with the velocity and thus having velocity and mass both defines their momentum..

Answer 7

they have an equivalent relativistic mass expressed by E=mc^2.

Answer 8

@Vaidehi09 You are wrong. As velocity of particle incerases mass increases. And this is given by the relation
\[m=\frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}\]
where \[m_0\] is the rest mass of the particle (i.e., mass measured when the particle is at rest)

v is the velocity of particle
c is the velocity of light.

From this relation we concluded that as v increases m increases.

As v tends to c mass tends to infinity. so any massive particle cannot move with the velocity of light. Only those particles can move with the velocity of light whose rest mass is zero.

Answer 9

@Vaidehi09 Photons are attracted by graivty due to their mass.

The "deformation of space" or "curvature of space" are dealt in "EINSTEIN THEORY OF GENERAL RELATIVITY". These thoughts are enitrely different concept.

Answer 10

@ramkrishna: I see. Thanks for correcting me. I have another question - can a particle with rest mass equal to zero, have some mass when in motion? So, mass at rest and mass in motion are two different quantities? and they can be different for the same particle? How so?